Nuclear control of mitochondrial gene expression, import of cytoplasmically-synthesized proteins into mitochondria and assembly of multisubunit proteins that contain both nuclear-encoded and mitochondrial-encoded polypeptides are examples of nuclear-mitochondrial interaction that occur during mitochondrial biogenesis. Cytochrome c oxidase in S. cerevisiae, which is composed of three mitochondrial-encoded subunits and six nuclear-encoded subunits, is a useful model for studies of nuclear-mitochondrial interaction and mitochondrial biogenesis. Genetic analysis in S. cerevisiae indicates that more than 37 nuclear gene products are specifically required for biogenesis of cytochrome c oxidase. In the studies proposed here, the functions of two of these gene products, encoded by the PET48 and PET120 genes, will be elucidated. PET48 and PET120 are required specifically for expression of the mitochondrial gene COX3, which encodes subunit III of cytochrome c oxidase. The aims of these studies are to: clone and determine the nucleotide sequences of the wild-type PET48 and PET120 genes; identify the PET48 and PET120 gene products and generate antibodies to them; characterize the expression and subcellular distribution of the PET48 and PET120 gene products; and identify the step of COX3 gene expression (transcription, transcript processing, translation or protein stability) that is defective in pet48 and pet120 mutant strains. These studies will lay the groundwork for determining the physiological role of the PET48 and PET120 genes in nuclear-mitochondrial coordination and in regulation of mitochondrial gene expression. Because mitochondrial gene expression is so novel in other respects, it is possible that the studies proposed here will uncover additional novel gene regulatory mechanisms. Finally, if either or both of the pet48 and pet120 mutant classes are blocked in translation of COX3 mRNA, these studies will provide insight into the wild-type mechanism of mitochondrial protein synthesis, which is currently only poorly understood.